Gas adsorption isotherms from composition and flow-rate transient times in chromatographic columns .3. Effect of gas viscosity changes

A method of determining binary adsorption isotherms from the flow-rate and composition responses of a packed column to a step change in input composition has been described previously by Mason & Buffham (1996 Proc. R. Soc. Lond. A452, 1263-1285). The method involves adding a small perturbation stream to a carrier gas mixture passing through a chromatographic column. The bulk of the observed effects arise from the small change in composition of the gas in the column caused by adding the perturbation gas. However, the increase in flow caused by adding the perturbation stream raises the pressure in the column and, under certain circumstances, this increase of pressure can cause significant adsorption (Mason & Buffham 1996 Proc. R. Soc. Lond. A452, 1287-1300). Apart from changing the amount of adsorption, the change in composition also alters the viscosity of the gas in the column and this too can affect the column behaviour. The change in viscosity is small but can have two consequences. The first is minor and is a small shift in the mean pressure in the column. The second effect is peculiar to the apparatus used in the previous experiments. A capillary flowmeter was used to monitor flow and capillary flowmeters respond to viscosity as well as flow. The effect of the change in viscosity in the capillary flowmeter is significant, even for gas mixtures with almost linear variation of viscosity with composition. Both effects can be corrected for by theory. The second effect might be removed by the addition of a delay line between the column and the flowmeter. A delay line is simply an empty tube which delays the arrival of gas of different viscosity and enables the capillary flowmeter to act as an ideal flowmeter for the limited period that the composition front is passing through, and leaving, the column. Precise isotherms for nitrogen-argon mixtures on 5A molecular sieve at 50 degrees C have been obtained and they are compared with binary Langmuir isotherms. The comparison is in terms of the gradients of the isotherms of each of the two components. Partial differentials of the amounts adsorbed in terms of the concentrations of nitrogen and argon are also obtained.